Sorry I'm a bit late to the party, BUT, I may have the definitive answer.
I suffer a lot with this rash. My wife bought me a Stainless steel Citizen Red Arrows watch. ?450!! Within a week I had the exact same rash. Luckily, having bought it online, she was able to return it under the distance selling regulations in the UK. She bought me the titanium version. Rash cleared up.
I had allergy tests at the Docs and nothing set it off. Anyways, after much trial and error, it was diagnosed as a reaction in the Stainless Steel. When you sweat, the salts in the sweat react with the stainless steel, which releases nickle, causing the rash. This can be exaggerated with a 'fitness' tracker, as invariably you will be sweating when wearing it.
I have not had the problem with the titanium watch.
There is a way of reducing the chance of the rash. The dermatologist told me to apply clear nail varnish!! Yes, really! I tried it on my Breitling, which has a stainless bracelet, and no issues! Could be tricky on the band though. I only got mine today (reduced prices rock!!), so I'm having a tinker with some kind of cover. If I can get the cut outs in the right place, I'll be chuffed. I've got some sheets of clear plastic, which, like laminating sheets, react to heat and stick firm.